CROSSOVER FROM LARGE TO SMALL POLARONS ACROSS THE METAL-INSULATOR-TRANSITION IN MANGANITES

We report Mn K-edge extended x-ray absorption fine structure spectra o n La0.75Ca0.25MnO3 up to high momentum transfer across the metal-insul ator (M-I) transition. The data show compelling evidence for (i) large , or intermediate Jahn-Teller polarons (IJTP), characterized by an ano malous longer Mn-O bond (Delta R = 0.09 Angstrom) in the metallic phas e (T < 170 K), and (ii) appearance of small JT polarons (SJTP) at T > 170 K, characterized by a longer Mn-O bond (Delta R = 0.21 Angstrom), which coexist with the IJTP above the M-I transition and has equal pro bability in the temperature range of colossal magnetoresistance.